1. Technical Field
The present invention is directed to an improved apparatus and method for protecting a probe card during an integrated circuit wafer sort sequence.
2. Description of Related Art
Probe cards are often used during testing, or sorting, of integrated circuits on wafers. The probe card contains the circuitry and probe elements necessary to test for various characteristics of the integrated circuits. Probe cards generally come in a cantilever type probe card or a vertical probe card type. A cantilever type probe card has the probe pins arranged nearly horizontally with bends approaching 90 degrees at the wafer end to allow contact with the integrated circuit pads. Vertical probe cards have probe pins which are, as the name implies, vertical and are contained in alignment blocks.
When integrated circuits are initially fabricated they are in the form of a wafer. A wafer is a round disc of a very thin material, usually silicon. The sizes of these discs currently range from 4 to 12 inches in diameter. Many hundreds or even thousands of individual devices, called die, will fit onto each wafer. The circuitry is layered one layer at a time on this disc until the devices are built. During this process, contact pads are also built which provide places for the power to be applied and the signals to move to and from each device.
Once the fabrication process is complete, each die must be tested, or sorted, to ensure it functions prior to packaging it into its"" final form, normally the black plastic devices we know as chips. In order to test the devices, the probe pins must be placed in contact with the contact pads on each device to allow the tester to power up the part and verify that it functions as desired. During sorting of the integrated circuit wafers, it is necessary to align the probe card with the integrated circuit wafer and bring the probes on the probe card into contact with the integrated circuit(s) on the wafer.
During this process, and sometimes during the actual testing process, a probe card is overdriven, meaning that the probe card is adversely driven into contact with the integrated circuit wafer or its"" mounting surface thereby causing damage to the probe card. Such overdriving of the probe card may be the result of mechanical malfunctions in the driving mechanism and/or operator error.
Processes and procedures have been devised to try and avoid such destruction of probe cards. The processes required to set up and operate the equipment which sorts the wafers are covered in very specific, step by step procedures which follow a logical sequence designed to protect both the probe cards and the wafers from damage. For instance, procedures are in place which require that the system be reset to a safe condition each time a probe card is changed. The software which operates the equipment also has many safeguards in place which help prevent machine and operator error from causing damage. Unfortunately, all circumstances cannot be accounted for in software, and software does not always perform as intended. The probe cards are designed in view of these processes and procedures. However, even with these processes and procedures, situations arise and/or mistakes occur which still result in the destruction of probe cards. Therefore, it would be beneficial to have an apparatus and method for protecting probe cards during sort sequences which is comparatively foolproof with regard to the known procedures and processes.
The present invention provides an apparatus and method for protecting a probe card during a sort sequence. The present invention provides a plurality of physical blocks, which could be of any shape, hereafter referred to as probe card protectors, attached to a bottom, or probe side of a probe card.
The probe card protectors are machined to a height that is less than the height of the probe tip and a guard band. The degree to which the probe card protectors are shorter than the height of the probe tip and guard band may be set based on an amount of overdrive of the probe card that is deemed to be acceptable. The acceptable wear of the probe card must also be taken into account when deciding on a guard band height.
The guard band is equal to allowable overdrive plus acceptable wear (from a new probe card to a worn probe card is normally on the order of 10 mils) plus a small amount to allow for any number of variables which may include heat, adverse probe tip wear patterns, excessive force during normal probe sequence, etc. The idea of the guard band is to place the probe card protectors as close as possible to the probe tip height yet low enough so as to never impact normal probing.
The height of the probe tip is measured at its"" lowest point where contact is made with the wafer contact pads, which corresponds to the probes farthest point from the bottom of the probe card. This height is hereafter referred to as probe card depth.
The probe card protectors are positioned on the probe card in positions where they will not interfere with the probe card docking mechanism, or prober, function. The prober is the name given to the equipment in which the probe card mounts and which manipulates the wafers in such a way as to position the probe tips into contact with the wafer contact pads, thereby allowing automated testing. The probe card protectors are machined from a base material that meets electrical and temperature requirements of probing.
During a sort sequence, if the probe card is overdriven due to mechanical or operator error, the probe card protectors of the present invention come into contact with the integrated circuit wafer after an acceptable amount of overdrive occurs. When the probe card protectors of the present invention contact the integrated circuit wafer, the contact action causes the z drive motor on the driving mechanism of the prober to stall, thereby stopping the driving of the probe card in the z direction. In this way, the driving of the probe card is stopped before damage to the probe card occurs.
Thus, the present invention provides a mechanical mechanism by which unacceptable overdrive of a probe card during a sort sequence or during the process of making the equipment ready to initiate a sort sequence is prevented. Because the solution of the present invention is of a mechanical nature rather than a procedural nature, the present invention operates to prevent damage to probe cards even in the presence of mechanical and/or operator error. Moreover, the present invention is relatively low cost compared to prior art software and sensor based approaches to solving the problem of probe cards being overdriven during sort sequences. These and other features of the present invention will be described in, or will become apparent to those of ordinary skill in the art in view of, the following detailed description of the preferred embodiments.